1. Field of the Invention
The present invention relates to an optical apparatus for image scanning for reading image information recorded on an original by irradiating light thereto and receiving either the light transmitted therethrough or the light reflected therefrom by a photoelectric conversion means.
2. Description of the Related Art
Heretofore, an image scanning apparatus has widely been used which photoelectrically reads image information recorded on an original such as a film and optionally processes the so-read image information to produce separation films for printing or to platemaking.
In an optical apparatus for image scanning used in this type of image scanning apparatus, a line sensor shown in FIG. 1, for example, has been used for the sake of the resolution capabilities and the reading rate. To utilize the line sensor, it is necessary to illuminate an elongated region extending along the longitudinal direction of the line sensor. It is therefore preferable to employ an elongated light source 2. In FIG. 1, reading light L is emitted from the light source 2 which is elongated in the direction (i.e., a main scanning direction) perpendicular to the sheet paper. The emitted light is focused, or concentrated in width in an auxiliary scanning direction perpendicular to the main scanning direction by a slit 4, onto an original F bearing image information recorded thereon. The original F is contained in a film cassette 6. In the film cassette 6, the original film F is placed between support glass plates 8a and 8b through which the reading light L passes. The film cassette 6 is moved in the direction indicated by the arrow X for auxiliary scanning. Then, the reading light L having passed through the original F is focused by a reading lens 10, and then introduced into a CCD line sensor 12. The CCD line sensor 12 has a plurality of sensor elements in an alignment along the main scanning direction, which converts the reading light into an electric signal corresponding to the image information.
The density of a color reversal film, which is normally used as an original, ranges in the order of 0 to 4.0 expressed in the optical density. In order to accurately measure a density of an original having an optical density of 2 or higher, it is necessary to suppress undesirable flare coming from other than pixels to be read to a level 1/1000 to 1/20000 times the level of the necessary image light.
The width of the sensor alignment of the CCD line sensor 12 corresponds to the width of one pixel (normally 7 .mu.m to 10 .mu.m) in an alignment of pixels extending in the auxiliary scanning direction (i.e., in the direction of arrow X). Thus, when the reading light L falls on a region other than the region where the photoelectric converter elements are, it tends to cause a flare which deteriorates the quality of the read image. In order to avoid such inconvenience, there is provided a slit 4 for limiting the width of the reading light L spreading in the auxiliary scanning direction.
It is necessary for a high-density original to be reproduced precisely to set the width of the reading light L to less than 50 times the width of the sensor alignment of the CCD line sensor 12, preferably less than 20 times, and more preferably less than 10 times the width thereof.
However, the reading light L transmitted through the slit 4 tends to be influenced by surface roughness of the original film in the auxiliary scanning direction, since the exit direction of the reading light L from the slit 4 is limited within a predetermined range and the numerical aperture of the slit 4 in the auxiliary scanning direction is small. A problem has been pointed out that a portion of the reading light L is scattered by the surface roughness of, for example, scratches or dust on the support glass plates 8a and 8b of the film cassette 6 or the original F, or non-glare glass used as the support glass plates 8a and 8b, and that this surface roughness clearly appear on the resultant image.